1. Field of the Invention
The present invention relates to design support of a print circuit board.
2. Description of the Related Art
As electronic equipment gains higher speed and more advanced functions, it handles a signal including high frequency components (to be referred to a high-speed signal hereinafter), and a problem of emission noise generated from the electronic equipment becomes obvious. As main generation causes of the emission noise, division and detour of a “return current” path induced in the neighborhood of a signal path of the high-speed signal are known.
In order to avoid division and detour of the return current path in the design stage of a printed circuit board, the return current path is simply derived and visualized based on CAD (Computer Aided Design) data of the printed circuit board. Then, a designer or checker of the printed circuit board visually checks division or detour of the return current path, and judges whether modifications of the conductive wiring patterns and the layout of parts are needed. If such modifications are necessary, the designer or checker modifies the conductive wiring patterns or the layout of parts.
As a method of deriving the return current path, the following techniques have been proposed.
The invention of Japanese Patent Laid-Open No. 2009-146271 (literature 1) defines, as nodes which are numbered along the signal lines, bent points of a signal line, vias for interlayer connections, and intersections between edges of a conductive wiring pattern and the signal line, where a return current is induced. The invention selects, from the respective nodes, those through which a return current path may run as return node candidates. Then, the invention derives a path defined by joining the return node candidates in a numerical order as a return current path.
The invention of Japanese Patent Laid-Open No. 2007-11629 (literature 2) selects a layer to be checked for each layer on which a signal line is wired (to be referred to as a signal layer hereinafter), and extracts a point where a return current path steps over a slit (non-conductive portion) on the selected layer. For each point where the return current path steps over the slit, the invention extracts a bypass capacitor and via located around that point, and checks whether or not the bypass capacitor and via can bypass a return current, in other words, whether or not a bypass exists. Then, the invention checks, based on information indicating a checking range, whether or not the bypass exists within the checking range, in other words, whether or not the bypass is effective.
The invention of Japanese Patent No. 3977638 (literature 3) detects an overlapping area where an enlarged area obtained by enlarging a line width of a signal line by a first coefficient and an area where a potential of each layer is fixed (for example, a ground pattern or power-supply wiring pattern) overlap each other. When the overlapping area is continuous in the enlarged area, a return current path which runs through that enlarged area is searched. On the other hand, when overlapping area is discontinuous in the enlarged area, a return current path, which detours, in a discontinuous portion, a range obtained by enlarging the line width of the signal line by a second coefficient larger than the first coefficient, and runs through the enlarged area in a continuous portion, is searched.
When a return current induced in the neighborhood of a signal line is divided by a slit on a layer (to be referred to as an adjacent layer hereinafter) which is adjacent to a signal layer, it normally flows through an area (to be referred to as a constant potential area hereinafter) where a potential of the signal layer is fixed.
The invention of literature 1 extracts a path which runs through a constant potential area of the signal layer as a candidate of the return current path between return node candidates. However, when the length of a path which detours the slit is shorter than the path which runs through the constant potential area, the invention adopts the path which detours the slit as the return current path. That is, the path which runs close to the signal line is not always preferentially selected as the path where a return current detours a divide portion.
The invention of literature 2 extracts a via and bypass capacitor around a divide portion of the return current path, and checks whether or not a bypass exists and whether or not the bypass is effective. However, the invention does not include any description about a practical bypass deriving method.
The invention of literature 3 does not always derive a return current path on the adjacent layer when there is a discontinuous point (that is, a divide portion of a return current path) in an overlapping area between the enlarged area and the constant potential area of the signal layer. That is, as in the invention of literature 1, a path which runs close to the signal line is not always derived as a path which detours the divide portion of the return current path.